Process for producing a pretreated feestock

ABSTRACT

A process for producing ethanol from a pretreated feedstock is provided. The feedstock is selected from grasses, cereal straws, stover, and combinations thereof, and least about 80% of the feedstock has a particle length of between about 2 cm and about 40 cm. This process comprises wetting the feedstock in liquid, pressing the wet feedstock through one roll press or a series of roll presses to remove at least a portion of water and soluble substances from the wetted feedstock and to shear the feedstock to produce feedstock particles of a size suitable for pumping at a solids concentration of about 8% to about 20% when slurried. At least one roll press, or at least one roll press in the series of roll presses comprises rolls with circumferential v-shaped grooves. The pressed feedstock particles are slurried to produce a slurried feedstock having a consistency of between about 8% and about 20%, and the slurried feedstock pumped into a pretreatment reactor. Dilute acid pretreatment of the slurried feedstock is carried out at a temperature of 160° C. to 280° C.

This application is a divisional of application Ser. No. 11/661,863filed Mar. 5, 2007, which is itself a 371 National stage application ofInternational application No. PCT/CA2005/001374, filed Sep. 9, 2005,which claims benefit of U.S. provisional application No. 60/609,098,filed Sep. 10, 2004, each of which are incorporated by reference herein.

The present invention relates to a process for producing a pretreatedfeedstock, more particularly to a process for producingcellulose-containing pretreated feedstock.

BACKGROUND OF THE INVENTION

Fuel ethanol is currently produced from feedstocks such as cornstarch,sugar cane, and sugar beets. However, the production of ethanol fromthese sources cannot expand much further due to limited farmlandsuitable for the production of such crops and competing interests withthe human and animal food chain. The use of fossil fuels, with theassociated release of carbon dioxide and other products, in theconversion process is a negative environmental impact of the use ofthese feedstocks

The production of fuel ethanol from cellulose-containing feedstocks,such as agricultural wastes, grasses, forestry wastes, and sugarprocessing residues has received much attention due to the availabilityof large amounts of these inexpensive feedstocks and the cleanliness ofethanol as a fuel compared to gasoline. In addition, a byproduct of thecellulose conversion process, lignin, can be used as a fuel to power thecellulose conversion process, thereby avoiding the use of fossil fuels.Studies have shown that, taking the entire cycle into account, the useof ethanol produced from cellulose generates close to nil greenhousegases.

The cellulosic feedstocks that may be used for ethanol productioninclude agricultural wastes such as corn stover, wheat straw, barleystraw, canola straw, and soybean stover. Grasses such as switch grass,miscanthus, cord grass, and reed canary grass may also be used.

Cellulose consists of a crystalline structure that is very resistant tobreakdown, as is hemicellulose, the second most prevalent component ofthese feedstocks. The conversion of cellulosic fibers to ethanolrequires liberating cellulose and hemicellulose from lignin orincreasing the accessibility of cellulose and hemicellulose within thecellulosic feedstock to cellulase enzymes, depolymerizing hemicelluloseand cellulose carbohydrate polymers to free sugars and, fermenting themixed hexose and pentose sugars to ethanol.

The feedstock is conveyed into the plant and the feedstock particles aretypically reduced to a desired size suitable for handling in subsequentprocessing steps.

Among well-known methods used to convert cellulose to sugars is an acidhydrolysis process involving the use of steam and acid at a temperature,acid concentration and length of time sufficient to hydrolyze thecellulose to glucose (Grethlein, 1978, J. Appl. Chem. Biotechnol.28:296-308).

An alternative method of cellulose hydrolysis is an acid prehydrolysis(or pre-treatment) followed by enzymatic hydrolysis. In this sequence,the cellulosic material is first pretreated using the acid hydrolysisprocess described above, but at milder temperatures, acid concentrationand treatment time. This pretreatment process is thought to increase theaccessibility of cellulose within the cellulosic fibers for subsequentenzymatic conversion steps, but results in little conversion of thecellulose to glucose itself. In the next step, the pretreated feedstockis adjusted to an appropriate temperature and pH and then submitted toenzymatic conversion by cellulase enzymes.

The hydrolysis of the cellulose, whether by acid or by cellulaseenzymes, is followed by the fermentation of the sugar to ethanol, whichis then recovered by distillation.

The efficient conversion of cellulose from cellulosic material intosugars and the subsequent fermentation of sugars to ethanol are facedwith major challenges regarding commercially viability. In particular,the feedstock particles are often too large to be efficiently handled orprocessed. One desirable type of handling system that requires smallparticles is pumping. In dry processes, for particle size reduction,water is not added to the feedstock. Dry processes which includegrinding, milling or crushing, require large amounts of power that addsto the cost of the overall process. Furthermore, dry processing to asmall particle size suitable for pumping is unlikely to be successfulfor feedstocks having high or variable moisture contents. Somefeedstocks containing 20% or higher moisture frequently blind themilling equipment, and it does not provide a sustainable or suitablesize reduction. The alternatives are wet grinding processes or apparatusthat make use of equipment such as using refiners or hydropulpers;however, wet grinding also requires costly equipment and high powerusages. Furthermore, wet grinding produces a material that is verydilute and costly to handle and process.

A second problem with the conversion process is that the acidrequirement in the pretreatment process is very high. Many feedstocks,such as straws and corn stover, contain a high native alkalinity thatrequires the addition of 0.5% to 6% w/w (of the feedstock) of sulfuricacid to achieve an efficient hydrolysis of the hemicellulose andexposure of the cellulose surface area. A significant amount of thisacid serves to offset the alkalinity inherent within the feedstock. Thishigh acid usage not only increases the cost of the process, but can alsocause degradation of the xylose and other products during thepretreatment process.

WO 02/070753 (Griffin et. al.) describes a leaching process comprisingcontacting the feedstock with water for at least two minutes to leachout the salts, protein, and other impurities, followed by removal of thewater and soluble compounds. The process of Griffin et. al. removesalkali from lignocellulosic feedstocks, thereby decreasing the acidrequirement for pretreatment. Griffin requires particle size reduction,but the processes consume a high level of power and, in combination withthe equipment required to carry out the leaching process, result inincreased overall process costs.

The use of presses for dewatering biomass is known in the art. Forexample, U.S. Pat. No. 4,436,028 (Wilder) describes the use of ahammermill to greatly reduce particle size followed by a two-roll millexerting severe pressure to decrease the moisture content of wastematerial. Similarly, U.S. Pat. No. 4,525,172 (Eriksson) teaches thedewatering of biomass using presses with sieving drums. However, thesemethods do not result in the grinding, shearing, or particle sizereduction of the biomass during pressing or dewatering. This results inhigh capital and operating costs without achieving the necessaryparticle size reduction.

U.S. Pat. No. 4,543,881 (Anderson) discloses an apparatus for dewateringpeat which includes an outer tubular roll and a smaller inner rollreceived in the outer roll. The smaller inner roll rotates so that itsouter surface moves along the inner surface of the outer tubular roll,thereby compressing peat placed between the rolls to effect dewatering.By operating the rolls at different speeds, shear forces and compressionforces act on the peat. However, the purpose of the roll compressionapparatus is to dewater the peat and not to reduce particle size.

U.S. Pat. No. 2,828,081 (Collins) describes the use of roll presses toseparate cork from phloem tissue on dry bark. A dried cork-rich fractionis passed through a differential speed roller mill that, throughshearing action, breaks up the cork aggregates without substantiallyreducing the cork particle size. This process is not designed for sizereduction of the particles of the material. As well, bark is notsuitable for ethanol production.

In order to address the need for further particle size reduction ofbiomass, various approaches have been taken. For example, U.S. Pat. No.6,036,818 (Odmark) describes a pulp dewatering device having two rollsthrough which the pulp is pressed (a roll press). As the pulp passesthrough the rolls, a doctor blade disintegrates and guides the pulp outof the press, and the pulp is further disintegrated by a screwdisintegrator. U.S. Pat. No. 5,451,296 (Pikulin), teaches the use of athickening unit (e.g. a twin roll press) to remove excess liquid fromlow consistency pulp. The resulting high consistency pulp is conveyed toa comminuting unit, such as a fJuffer, to generate pulp particles of 10mm or less. The roll presses used in either U.S. Pat. No. 6,036,818 orU.S. Pat. No. 5,451,296 do not convey grind, shear, or reduce theparticle size of the pulp. Rather, additional equipment is required toaccomplish particle size reduction, thus increasing both equipment andenergy costs for the overall process.

U.S. Pat. No. 4,728,044 (Duill and Brummer) discloses a system forgrinding and drying damp initial material. The starting material ispre-comminuted by a hammer mill while being dried with hot gas.Following further drying in a rising main, the material is furthercomminuted in the nip between the rollers of a high-pressure roll. Thefinished material emerges as dried and ground raw material. Although theprocess is suitable for the grinding and drying of materials such as rawcement meal, cement clinker, ore, coal and the like, the furtherprocessing of feedstock to produce ethanol is not addressed. None of thesuitable feedstocks for ethanol production are mentioned.

U.S. Pat. No. 4,237,226 (Grethlein) describes milling of dry oak woodchips in a laboratory setting using a Wiley mill to produce asawdust-like product. The ground chips pass through a screen of 60 mesh,then are slurried in water at a ratio of water to solids of 18.5 to 1 byweight prior to feeding the slurry to a continuous pretreatment reactor.The Wiley mill is not suited for use with fiber with over 20% moisturecontent, and exhibits high power consumption. Furthermore, there is nodisclosure of commercial-scale equipment that may be used to carry outthese processing steps.

Millett et al. (Biotechnol. & Bioeng. Symp. No. 6 (1976) 125-153)disclose several physical treatments for the preparation of feedstocks,including dry ball milling, wet ball milling and vibratory ball milling.The production of fine particles by dry ball milling adds substantiallyto the cost of the process, while wet ball milling for 72 hoursincreased the digestibility of cellulose by rumen bacteria. However, 72hours is not a practical treatment time in a production process, andthere is no mention of subsequent pretreatment or enzymatic hydrolysis.Vibratory ball milling of dry spruce and aspen chips for 30 minutes at220° C. was found to increase the rate of enzymatic hydrolysis. However,this treatment adds considerable expense to the process.

U.S. Pat. No. 3,554,453 (Thale et al.) discloses an apparatus forshredding fibrous articles such as groundwood, compressed webs and flatpieces of sulfite and semi-chemical pulp. The apparatus contains ashredding roller and a holding roller, each with interdigitating tootheddiscs for shredding the fibrous material as it advances between therollers. The action of the toothed discs on the rollers generatesdefibered material and does not result in pressing of the material.

U.S. Pat. No. 4,683,814 (Plovanich et al.) discloses an apparatus and adry process for dewatering cellulosic biomass, which utilizes a pair ofsmooth opposed rolls operating at different speeds. Due to thedifferential roller speeds, the compressed biomass is heated, whichresults in additional moisture removal, and particle size reduction.Furthermore, moisture collects on the roll rotating at the higher rateand compressed material adheres to the roll rotating at the lower rate.This allows moisture to be collected from the roll rotating at thehigher speed and compressed material to be collected from the rollrotating at the lower speed. Although the process provides an effectivemeans for the dewatering of biomass, the process of Plovanich et al.only removes a minority of the alkalinity of the feedstock. Moreover,Plovanich et al. do not teach the production of a cellulosic feedstockhaving a particle size suitable for pumping.

The process for extracting sugar from sugar cane feedstocks is wellknown. This involves washing the sugar cane surface to removeimpurities, coarsely chopping the stalks into smaller pieces, andcrushing the sugar cane pieces in a series of roller mills to extractthe juice. The juice is collected from the presses and further processedto produce sugar. The residue from the cane stock after juice extraction(bagasse) is usually burned at the mill.

In order for a continuous pretreatment of cellulosic feedstocks to beeconomically and commercially viable, the pretreatment system must beamenable to the pretreatment of a variety of feedstocks; the alkalinityof the feedstock must be reduced from its native levels, so as todecrease the acid requirements and degradation of sugar products byacid; and the feedstock particle size must be reduced, without requiringexcessive power or capital equipment, such that the particles can bepumped in aqueous medium.

The development of such a system remains an important component of theoverall process to convert cellulosic feedstocks to glucose andsubsequently to ethanol.

SUMMARY OF THE INVENTION

The present invention relates to a process for producing a pretreatedfeedstock, more particularly to a process for producingcellulose-containing pretreated feedstock.

It is an object of the present invention to provide a process forproducing a pretreated feedstock with improved efficiency.

The present invention provides a process for producing a pretreatedfeedstock, the process comprising the steps of:

-   -   a) providing a feedstock selected from the group consisting of        grasses, cereal straws, stover, and combinations thereof,        wherein at least about 80% of the feedstock has a particle        length of between about 2 and about 40 cm;    -   b) wetting the feedstock in an aqueous stream to about 0.25 to        about 10 times the maximum water holding capacity of the        feedstock to produce a wet feedstock;    -   c) pressing the wet feedstock through one roll press or a series        of roll presses to remove at least a portion of water and        soluble substances from the wet feedstock and to shear the wet        feedstock to produce a pressed feedstock having a particle size,        such that when the pressed feedstock is slurried to produce a        slurried feedstock, the slurried feedstock is capable of being        pumped at a solids concentration of about 8% to about 20%,    -   wherein the one roll press, or one or more than one roll press        in said series, comprises rolls with circumferential v-shaped        grooves, and    -   wherein the pressed feedstock has a consistency of at least        about 35% dry solids after passing through a nip point in the        one roll press or a nip point in one or more than one roll press        in said series;    -   d) slurrying the pressed feedstock particles to produce a        slurried feedstock having a solids concentration of about 8% to        about 20% and pumping the slurried feedstock into a pretreatment        reactor; and    -   e) carrying out dilute acid pretreatment of the slurried        feedstock, at a temperature of about 160° C. to about 280° C. to        produce the pretreated feedstock.

The present invention is also directed to the method as just described,wherein, after the step of slurrying (step d), the feedstock ispretreated at a temperature of about 170° C. to about 260° C. and at pHof about 0.8 to about 2.0 for a period of 0.1 to 30 minutes.Furthermore, the pretreated feedstock may be hydrolyzed by cellulaseenzymes to produce glucose, which may subsequently be fermented toethanol. Prior to the step of pressing (step c), the feedstock may bepartially leached.

The present invention is also directed to the invention as describedabove, wherein, in the step of providing (step a), the cereal straw isselected from the group consisting of wheat straw, barley straw, ricestraw, canola straw, and oat straw, and the stover is selected from thegroup consisting of corn stover and soybean stover. Preferably, in thestep of providing (step a), at least about 80% of the feedstock has aparticle length of between about 2 and about 30 cm.

The present invention is also directed to the invention as describedabove, wherein, in the step of pressing (step c), the one roll press orone or more than one roll press in the series creates additional shearto reduce the feedstock particle size, and wherein the additional shearis created by a difference in diameter or a difference in speed of therolls in the one roll press, or in one or more than one roll press inthe series.

Preferably, the series of roll presses comprises 3 roll presses,although 2 roll presses can be used as well.

The present invention is also directed to the invention as describedabove, wherein a series of presses are used, and wherein the step ofwetting (step b) comprises countercurrent washing of the feedstock withpressate collected from at least one roll press in the series.

The present invention is also directed to the invention as describedabove, wherein, in the step of wetting (step b), the liquid is at atemperature of between about 20° C. and about 95° C., or between about30° C. and about 85° C.

The present invention is also directed to the invention as describedabove, wherein, in the step of slurrying (step d), the slurriedfeedstock has a consistency of between about 10% and about 18% drysolids, or between about 12% and 15% dry solids. Preferably, after thestep of pressing (step c) and before the step of slurrying (step d), thepressed feedstock has a consistency of at least about 35% dry solids. Inthe step of slurrying (step d), at least about 70%, 80% or 89% of thefeedstock particles may be 2.4 cm or less in length.

The present invention is also directed to the invention as describedabove, wherein, in the step of pressing (step c), one or more than oneother press or one or more than one other dewatering device is used incombination with the one roll press or the series of roll presses,wherein the one or more than one other press or the one or more than oneother dewatering device is not a roll press. If other press types ordewatering devices besides roll presses are used in combination with theone roll press or the series of roll presses, the consistency of thefeedstock after pressing the wetted feedstock is preferably at leastabout 35% dry solids.

Furthermore, the present invention is also directed to the invention asdescribed above, wherein the process is a continuous process withcontinuous feeding of the feedstock and continuous withdrawal of thepretreated feedstock.

The wet treatment process of the present invention overcomes thedisadvantages of the prior art, as it requires much less power inputthan a dry grinding process. The invention also mitigates the risk ofblinding dry milling equipment with feedstock having a high moisturecontent. Furthermore, the presses require much less power input and costmuch less than other wet grinding equipment, such as refiners orhydropulpers. The process of the present invention produces a feedstockthat is pumpable with a minimal amount of aqueous solution.

In addition, by using a wet process, a significant portion of the salts,alkali, and protein are removed from the feedstock. This decreases theacid requirement and potentially increases the xylose yield. Thesebenefits are consistent with those achieved by WO 02/070753 (Griffin)using a leaching of the feedstock, but without the contact times andequipment used in a leaching process.

Therefore, the invention offers significant advances in the productionof sugar from lignocellulosic feedstocks.

This summary of the invention does not necessarily describe all requiredfeatures of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparent fromthe following description in which reference is made to the appendeddrawings wherein:

FIG. 1A shows an example of a feedstock handling system in accordancewith the present invention.

FIG. 1B shows a roll press comprising three rollers.

FIG. 1C shows a cross section of the tooth profile that may be presenton the outer surface of the rollers in the roll press.

FIG. 2 shows results of an analysis of wheat straw particle lengthfollowing processing according to the present invention. Particle lengthmeasurements were determined following coarse chopping after a single(left bar), double (middle bar) or triple (right bar) pass through themill. 100 fibers were randomly selected and manually measured with aruler to estimate the particle length distribution.

FIG. 3 shows results of an analysis of wheat straw particle sizefollowing processing according to the present invention. Particle sizeis determined using standard square mesh sieves. The sieves separate thestraw on the basis of the particle diameter, not length. Three sampleseach of the coarsely chopped (left bar), single (left-middle bar),double (right-middle bar), and triple (right bar) milled material werepassed through sieves of varying sizes, and the results averaged foreach type of material.

FIG. 4 shows the amount of water-soluble potassium and total nitrogen(total Kjedahl nitrogen; TKN) found in the dry feedstock (left bar) orafter single (left-middle bar), double (right-middle bar) or triple(right bar) passes through the mill. The dissolved components weredetermined before and after pressing and are expressed as a percent ofthe original content.

FIG. 5 shows the medium consistency (MC®) pump current during testing ofsingle pass wheat straw.

FIG. 6 shows the MC® pump current during testing of combined double andtriple pass wheat straw.

DESCRIPTION OF PREFERRED EMBODIMENT

The present invention relates to a process and apparatus for handlingwet feedstock, more particularly to a process and apparatus for handlingwet cellulose-containing feedstock.

The present invention provides a process that allows for the crushingand shearing of feedstock and the removal of much the soluble salts,protein, sugars, alkaline compounds, and organic acids from thefeedstock. The wetted feedstock undergoes pressing using one roll press,or a series of roll presses. The result is that the particle size isreduced and moisture is pressed out of the wetted feedstock. Solublecomponents thus released may be collected during pressing and shearingof the feedstock and the pressed and sized feedstock can then besubjected to pretreatment or pretreatment combined with subsequenttreatment steps for its conversion to sugars. The process may be used asa continuous process, so that the feedstock may be fed, and preparedfeedstock withdrawn, in a continuous manner. However, the process mayalso be used for batch processing. The process of the present inventionremoves potential inhibitors from the feedstock, and also decreases theacid requirement during pretreatment.

The present invention provides a process for producing a pretreatedfeedstock comprising:

-   -   a) providing a feedstock selected from the group consisting of        grasses, cereal straws, stover, and combinations thereof,        wherein at least 80% of the feedstock has a particle length of        between about 2 and about 40 cm;    -   b) wetting the feedstock in an aqueous stream to about 0.25 to        about 10 times the maximum water holding capacity of the        feedstock to produce a wet feedstock;    -   c) pressing the wet feedstock through one or a series of roll        presses to remove at least a portion of water and soluble        substances from the wet feedstock and to shear the wet feedstock        to produce a pressed feedstock having a particle size such that,        when the pressed feedstock is slurried to produce a slurried        feedstock, the slurried feedstock is capable of being pumped at        a solids concentration of about 8 to about 20%,    -   wherein the one roll press, or one or more than one roll press        in said series, comprises rolls with circumferential v-shaped        grooves, and    -   wherein the pressed feedstock has a consistency of at least        about 35% dry solids after passing through a nip point in the        one roll press or a nip point in one or more than one roll press        in said series;    -   d) slurrying the pressed feedstock particles to produce a        slurried feedstock having a solids concentration of about 8% to        about 20% and pumping the slurried feedstock into a pretreatment        reactor; and    -   e) carrying out dilute acid pretreatment of the slurried        feedstock at a temperature of about 160° C. to about 280° C. to        produce the pretreated feedstock.

The process of the invention is effective on a wide variety offeedstocks, including: (1) stover, such as corn stover or soybeanstover; (2) cereal straws, such as wheat straw, barley straw, canolastraw, oat straw, and rice straw; (3) grasses such as switch grass,miscanthus, cord grass, and reed canary grass; and any combinationthereof. These feedstocks are available in large quantity at low costand contain high levels of carbohydrates, which correspond to a highpotential ethanol yield.

The feedstock is usually conveyed into the ethanol plant in bales orother convenient form. The bales may be coarsely broken up to createparticles that can be handled by the one roll press or the series ofroll presses using any method known in the art. For example, thefeedstock may be broken up in a coarse size reduction process using ahammer mill, a rotary shredder, shear shredder, knife hog, tub grinder,wood chipper-like device, or any other device that reduces the particlesize of the entering solids. The size of the reduced feedstock issuitable for handling in the system of the present invention, i.e., atleast 80% of the reduced feedstock has a particle length of about 2 toabout 40 cm, or any amount therebetween. Preferably, at least 80% of thefeedstock may have a length of about 2 to about 30 cm, or about 4 toabout 25 cm, or any amount therebetween.

In the process of the present invention, the feedstock is wetted in anaqueous stream prior to pressing. The aqueous stream can be any suitableliquid that wets the feedstock and thereby permits it to be pressed bythe one or more roll presses. For example, the liquid may be water, millwater, or recycled wash liquor (pressate) obtained from earlierfeedstock processing. In the latter case, wetting of the feedstock maycomprise counter-current washing of the roll presses with the resultingliquor used to wet the feedstock upon entry into the plant.

The feedstock may be wetted using any method known in the art. Forexample, the feedstock may be wetted by spraying the liquid onto thefeedstock, by immersion in a tank of liquid, or by passing the feedstockthrough a tank of liquid.

The amount of liquid used is chosen so as to be enough to wet thefeedstock and provide some extra liquid for the removal of impurities,but is not in such excess as to produce dilute streams that increasehandling expenses. The optimal amount of liquid required to adequatelywet the feedstock will vary based on the physical properties of thefeedstock and the feedstock particle size. An acceptable amount of wateris about 0.25 to about 10 times the maximum water holding capacity ofdry feedstock, or any amount therebetween. For example, the amount ofwater added may be about 0.25, 0.5, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0, 6.0,7.0, 8.0, 9.0 and 10 times the maximum water holding capacity of dryfeedstock, or any amount therebetween. More preferably, the amount ofwater added is about 1.5 to about 3 times the maximum water holdingcapacity of dry feedstock, or any amount therebetween. The maximum waterholding capacity of a feedstock is determined by measuring the weight ofwater which is absorbed by a known mass of loosely packed feedstockuntil the point at which additional water added to the feedstock is freewater. This point is measured as the point in which the water forms athin continuous layer over the feedstock. In determining the maximumwater holding capacity of a feedstock, it is preferable that thefeedstock is mechanically disrupted into particles of about the samesize. Furthermore, as would be evident to a person skill in the art, itis preferred that the maximum water holding capacity of a feedstock bedetermined on a loosely packed and not tightly packed feedstock. As anexample, if 1 kg of feedstock (dry basis) holds 4000 g of water, themaximum water holding capacity of the feedstock is 4000 g/kg. Theacceptable amount of water is then 1000 g to 40,000 g per kg feedstock.

Alternatively, an acceptable ratio of water to solids may be from about2.5:1 to about 100:1, by weight, or any amount therebetween. Morepreferably the ratio of water to solids is from about 2.5:1 to about25:1, by weight, or any amount therebetween. For example the ratio ofwater to solids may be about 2.5:1, 5:1, 7.5:1, 10:1, 12.5:1, 15:1,17.5:1 20:1, 22.5:1 or 25:1, by weight, or any amount therebetween. Atypical ratio of liquid to feedstock is about 2.5:1 to about 10:1, orany amount therebetween, for example 2.5:1, 5:1, 7.5:1, or 10:1 byweight.

The addition of water followed by pressing removes a significant amountof the alkalinity and nitrogen and potassium from the feedstock. Thisdecreases the acid usage in pretreatment and may increase the xyloseyield.

The feedstock may be wetted as part of a sand removal process or aleaching process prior to pressing. Sand removal has the advantage ofpreserving the roll presses from abrasion due to the presence of sand onthe feedstock. For sand removal, water is combined with the feedstock atthe ratio as described above, i.e., about 0.25 to about 10 times themaximum water holding capacity of dry feedstock, or any amounttherebetween, more preferably about 1.5 to about 3 times the maximumwater holding capacity of dry feedstock, or any amount therebetween. Inthis process, feedstock wetting may be carried out over a rotating drumwith a screen, or some other equipment familiar in the art. Thefiltrate, which contains sand, water, and soluble impurities, is settledto remove the sand, while the liquor can be reused to wash feedstocknewly conveyed to the plant. In a non-limiting example, sand removal iscarried out by introducing the sand-containing filtrate to a hydroclone.

Regardless of whether sand is removed or not, the temperature of theliquid for wetting the feedstock may be any temperature in the range ofabout 20° C. to about 95° C., for example a temperature of 20, 25, 30,35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90 or 95° C., or any amounttherebetween. In general, warmer temperatures may result in a moreefficient extraction of soluble components by the liquid. Thetemperature range of the liquid may be about 30° C. to about 85° C., orany temperature therebetween.

The wetted feedstock is then subjected to size reduction. This isaccomplished by conveying the feedstock through one or a series of rollpresses. Each roll press used in the process of the present inventionmay be a roll press that is commercially available (e.g. from EdwardsEngineering Corp. Houston Tex.; Bundaberg Foundry Engineers Ltd.,Bundaberg Australia; or Fulton Iron and Manufacturing LLC, St. LouisMo.), and may comprise a suitable number of rolls, for example, but notlimited to, three (see for example FIG. IB) or four rolls (see FIG. IA;roll presses 40, 50 and 60). Additionally, the roll press may be amodified roll press as described in more detail below.

The present invention may use either one, or more than one roll press inseries, to deliver adequate particle size reduction. For example, theprocess of the present invention may use from one to seven roll presses,or any amount therebetween; for example, 1 or 2, 3, 4, 5, 6 or 7 rollpresses in series, may be used. In a more specific example, the processuses two to four roll presses in series. Preferably, the process usestwo or three roll presses in series. The roll presses in the series maybe arranged one behind the other in succession (see FIG. IA), or otherprocess equipment may be interposed between the presses.

Furthermore, one or more than one other types of presses or dewateringdevices could be used in combination with the one roll press or theseries of roll presses. Such press types or dewatering devices mayinclude, but are not limited to, belt presses, filter presses, screwpresses and centrifuges.

Without wishing to be limiting in any manner, each roll press (3) mayhave three or four rolls (FIG. IB). In this configuration, three of thefour rolls are set in a triangular pattern with first and second lowerrolls (2A and 2B, respectively) at the bottom, side by side, and a thirdtop roll (4) at the top between the bottom rolls. Preferably, the firstand second rolls (2A, 2B) are supported by bearing journals fixed to thesupporting frame of the roll press, and the top roll (4) may be mountedon a set of guide plates that allows the roll (4) to move up and down.The top roll (4) may also have hydraulic cylinders mounted on each endfor application of the crushing, shearing or crushing and shearing forcebetween the top and bottom rolls. If the roll press comprises fourrolls, then the fourth roll acts as a feed roller. A series of rollpresses, each comprising four rolls, is shown in FIG. IA.

Referring to FIG. IB, in a non-limiting example, during operation of thepress (3), the wet feedstock is crushed twice, once between the top roll(4) and the first lower roll (2A) and then between the top roll (4) andthe second lower roll (2B). The feed opening (8A) between the top roll(4) and the first lower roll (2A) and the discharge opening (8B) betweenthe top roll (4) and the second lower roll (2B) are each set to adesired clearance. The feedstock is guided through the press by guidancedevice (9).

The rolls of the press may be constructed of any suitable material, forexample, but not limited to, stainless steel. The dimensions of eachroll may vary as required, but may, for example, which is not to beconsidered limiting, be from about 0.75 m to about 4 m long, or anyvalue therebetween, with a diameter of from about 0.45 m to about 2 m,or any value therebetween.

One or more than one of the presses comprise press rolls withcircumferential grooves having a “vee” shape (v-shape) cross-section cutinto the outer surface of each roll (6; FIG. 1C). The provision of rollpresses with such circumferential grooves enhances drainage and shearingof the wet feedstock. The moisture content of the wet feedstock isreduced, thereby removing soluble substances and increasing theconsistency of the feedstock. The shearing of the feedstock by thegrooves allows for efficient particle size reduction to produce apumpable feedstock.

The spacing (5) between the grooves may be from about 1.0 cm to about10.0 cm, or any value therebetween, and the groove depth may be fromabout 1.0 cm to about 10.0 cm, or any value therebetween. The top roll(4) is set such that the grooves of the three rolls mesh with eachother, but the rolls are not in contact (8A and 8B; FIG. 1B). Theclearance between the roll nip points may be from about 0.0001 cm toabout 0.1 cm, or any value therebetween. Meshing of the grooves (6) andcrushing load between rolls provides the majority of shearing action.

It should be appreciated that, if a series of presses are utilized, inorder to produce the required particle size reduction, it is notnecessary that all of the presses contain rolls with circumferentialgrooves. For example, it is contemplated that adequate shearing andmoisture reduction could be achieved if one or more of the pressescomprise rolls having circumferential grooves, while the remaining rollpresses have smooth rolls.

The rolls of the press apply pressure to the feedstock as it is fedthrough the roll press(es). The pressure applied to the rolls may beadjusted as necessary to create additional shear to reduce particlesize. A suitable pressure range for removal of the pressate andreduction of feedstock particle size is in the range of about 2400 toabout 3000 psi or any value therebetween. For example, the press nippressure may be of about 2400, 2500, 2600, 2700, 2800, 2900 or 3000 psi,or any value therebetween.

The roll presses may be modified to increase the degree of milling ofthe feedstock. Increased milling of the feedstock may be achieved byusing rolls of varying diameter rotating at the same speed. Thedifference in diameter may be of about 1.2 cm to about 20.0 cm, or anyvalue therebetween; more specifically, the diameters may differ by about1.5 cm to about 10.0 cm, or about 1.5 to about 2.5 cm, or any amounttherebetween. Alternatively, rolls of the same diameter may be used, butthe rolls are rotated at different speeds. In this case, the differencein speed between the rollers may be from 0% to about 10%, or any valuetherebetween; in a more specific example, the speeds may differ by about3.5% to about 10%, or any value therebetween. The use of rollers ofdifferent diameters, rollers rotating at different speeds, or the use ofboth rollers having different diameters and rotating at different speedsresults in efficient feedstock particle size reduction, and may resultin the use of fewer roll presses to achieve the desired feedstockparticle size. Furthermore, the gap between the rolls may be adjusted asdesired to create additional shear.

As a result of passing through one roll press or a series of rollpresses, the feedstock particle size is reduced to a size suitable forhandling. The particles are of a size such that, when the pressedfeedstock is subsequently slurried, the resulting slurry can be pumpedat a dry solids concentration of about 8 to about 20%, or any amounttherebetween. Preferably, the particles are of a size such that theslurried feedstock can be pumped at a solids concentration of about 10%to about 18% dry solids or any range therebetween, or about 12% to about15% dry solids, or any range therebetween. Preferably, the feedstock isof a size appropriate to be slurried and pumped using medium consistencypumps (such as a Sulzer MC® Pump). Once the feedstock exits one or aseries of roll presses, the majority of the feedstock particles may beabout 10 cm or less in length. Preferably, at least about 70%, 80% or89% of the feedstock particles are of a length of 2.4 cm or less. Forexample from about 70% to about 89%, or any amount therebetween, of thefeedstock particles are of a length from about 0.05 to about 2.4 cm orany amount therebetween.

As set forth above, pressing of the wetted feedstock results in anincrease in the consistency of the feedstock. If only one roll press isused, the consistency of the feedstock is at least about 35% dry solids,or from about 35% to about 95% or any amount therebetween dry solids,after passing through one nip point of the press. For example, theconsistency of the feedstock may be from about 35% to about 60%, or anyamount therebetween If a series of roll presses are used, then theconsistency of the dry solids is at least about 35%, or from about 35%to about 60%, or from about 35% to about 95%, or any amounttherebetween, dry solids, after passing through a nip point of at leastone (i.e., one or more than one) roll press in the series. By way ofillustration, and with reference to FIG. IB, the consistency of thefeedstock after passing between the top roll (4) and the first bottomroll (2A) or after passing between the top roll (4) and the secondbottom roll (2B) is at least about 35%. Preferably, after the step ofpressing and before the step of slurrying, the consistency of thefeedstock is at least about 35%. For example, with reference to FIG. IA,the consistency of the feedstock is at least about 35% after exiting thethird roll press, for example at 60.

It should be appreciated that, after the moisture content of the wettedfeedstock is reduced in a roll press, or other type of press ordewatering device which is optionally used in combination with the onepress or series of roll presses, the moisture content of the feedstockmay be increased prior to entering a downstream press or dewateringdevice. For example, an aqueous stream may be added to the discharge ofthe upstream press, the inlet of a downstream press, or dewateringdevice, or any point in between.

As a further step after pressing or slurrying, the feedstock may besubjected to additional leaching. This is carried out by contacting thefeedstock with water for a period of time adequate to allow a portion ofthe soluble components in the feedstock to dissolve, followed byseparating at least a portion of the aqueous solution from the feedstocksolids.

Following pressing, the feedstock is slurried in water. The amount ofwater added to the feedstock is typically chosen as the minimum thatallows the feedstock to be pumped. This is typically to achieve aconsistency or solids concentration of about 8% to about 20%, or anyvalue therebetween. For example, the consistency or solids concentrationmay be about 8, 10, 12, 14, 15, 16, 18, or 20%, or any valuetherebetween. Preferably, the reduced feedstock is slurried to produce afeedstock slurry having a consistency of about 10 to 18%, or about 12 to15%, or any range therebetween.

Alternatively, the amount of water added to the feedstock may be fromabout 2.5 to about 10 parts water per part solid, by weight, or anyvalue therebetween. For example, the ratio between water to solid, byweight, may be from about 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, 5:1, 5.5:1,6:1, 6.5:1, 7:1, 7.5:1, 8:1, 8.5:1, 9:1, 9.5:1 or 10:1 or any amounttherebetween. For example which is not to be considered limiting, thereduced feedstock is slurried in 5 or 6 parts water per part solids (5:1or 6:1).

The slurried feedstock is then subjected to a dilute acid pretreatmentprocess. The pretreatment process converts hemicellulose, or a portionthereof, to sugars and may convert a portion of the cellulose to sugar.A non-limiting example of such a treatment includes steam explosion, asdescribed in U.S. Pat. No. 4,461,648 (Foody; which is incorporatedherein by reference). Generally, dilute acid pretreatment conditions forlignocellulosic feedstocks comprise a temperature in the range of about160° C. to about 280° C., or any amount therebetween, for example 160,170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270 or 280° C., for aperiod of about 0.1 to about 30 minutes, or any amount therebetween,including 0.1, 0.5, 1, 5, 10, 15, 20, 25, or 30 min, and at a pH ofabout 0.8 to about 2.0, or any amount therebetween, for example a pH of0.8, 1, 1.2, 1.4, 1.6, 1.8 or 2.0. The low pH for pretreatment requiresthe addition of acid to the feedstock. The acid used for pretreatmentmay be any type of suitable acid known in the art, including, but notlimited to, sulfuric acid, sulfurous acid, or phosphoric acid. Sulfuricacid is preferred due to its low cost and, following recovery, its usein fertilizer in the form of sulfate salts. Prominent pretreatmentprocesses carried out with dilute acid include those described byGrethlein (U.S. Pat. No. 4,237,226) and in Sassner et al. (Appl.Biochem. Biotech, 2005, 121-124:1 101-1 117), which are incorporatedherein by reference.

The pretreated feedstock may then be subjected to enzymatic hydrolysiswith cellulase enzymes, to convert the cellulose to glucose. Withoutwishing to be bound by theory, the pretreatment increases thesusceptibility of the feedstock to hydrolysis by cellulase enzymes.Cellulase enzymes can typically tolerate a range of pH of about 3 to 7;therefore, the pretreated feedstock is generally neutralized prior toenzymatic hydrolysis. Adjustment of the pH after pretreatment can becarried out using ammonia gas, ammonia dissolved in water (ammoniumhydroxide), sodium hydroxide, lime, sodium carbonate or potassiumhydroxide. A pH more favorable to the cellulase enzymes is, for example,within the range of about 4.5 to about 5.0, or any value therebetween,for example a pH of 4.5, 4.6, 4.7, 4.8, 4.9 or 5.0. The pH-adjusted,pretreated feedstock can then be subjected to enzymatic hydrolysis usingcellulase enzymes.

By the term “cellulase enzymes”, “cellulase”, or “enzymes”, it is meantenzymes that catalyse the hydrolysis of cellulose to products such asglucose, cellobiose, and other cellooligosaccharides. Cellulase is ageneric term denoting a multienzyme mixture, produced by a number ofmicroorganisms, comprising exo-cellobiohydrolases (CBH), endoglucanases(EG) and β-glucosidases (βG). Among the most widely studied,characterized, and commercially produced cellulases are those obtainedfrom fungi of the genera Aspergillus, Humicola, and T{acute over (η)}choderma, and from the bacteria of the genera Bacillus and Thermobifida.The cellulase enzymes may be produced by genetic engineering techniques,which are well-known to those of skill in the art. In a non-limitingexample, the pretreated feedstock described above may be submitted tohydrolysis by cellulase enzymes produced by Trichoderma.

In a preferred embodiment, the sugar is fermented to ethanol.Fermentation may be carried out by yeast, bacteria or other microbes, oran enzyme mixture, capable of fermenting the product stream to a desiredefficiency and yield. In a preferred embodiment, the fermentation iscarried out using a genetically engineered yeast, for example, but notlimited to, Saccharomyces or Pichia, or bacteria, for example, but notlimited to, Zymomonas or E. coli capable of fermenting the pentosesugars xylose, arabinose, or a combination thereof, in addition to thehexose sugars glucose, mannose, galactose, or a combination thereof.Alternatively, the sugar in the product stream is fermented to lacticacid. Those skilled in the art are familiar with the requirements infermentation of sugar to produce ethanol, organic acids, or sugaralcohols.

With reference to FIG. IA, a non-limiting example of the overall processsystem is shown. The feedstock, for example bales of wheat straw areshredded and wetted with water at 0.25 to 10 times the maximum waterholding capacity of the wheat straw. The feedstock is washed over aninclined rotating drum with a screen (20). Sand and liquor containingsoluble solids pass through the screen. The stream of sand and liquor issent to a hydroclone or settling tank (30) to remove the sand. Theliquor may be reused to wash incoming feedstock, or processed for otheruses.

The screened, wetted feedstock is moved onto a first conveyor belt (1).The feedstock may be wetted by liquor from the pressate (43, 70 or both)of the first or second or both, roll press (40, 50), respectively. Thewet feedstock is conveyed to the first roll press (40), where it iscrushed and pressed. The pressate (43), which contains small particlesand liquor, may be sent (via line 45) to the inclined rotating drum witha screen (20) via first pump (48). Prior to being sent to the rotatingdrum (20), the pressate (43) may be sent to a first swirl tank (47). Thepressate solids are recovered and combined with the feedstock feedinginto the first roll press (40). The liquor, which contains solubleprotein, sugars, and salts, may be collected and potentially used as abyproduct. The liquor can also be sent for further processing, forexample, but not limited to, protein recovery and concentration.

The once-pressed solids are conveyed by a second conveyor (42) to thesecond roll press (50). The solids are wetted by pressate (80) from athird roll press (60). The wet solids pass through roll press (50). Thepressate (70), which contains small particles and liquor, is fed bysecond pump (56) to a solids removal screen (75) to recover solids. Thepressate (70) may pass through a second swirl tank (54) before beingintroduced to the solids removal screen (75). The pressate solidsrecovered by the screen (75) are fed to a conveyor (77) and introducedto the first conveyor (1) feeding the first roll press (40). The liquor(82) may be introduced to a washer liquor tank (92) and pumped to aclarifier (94) to remove fines. The clarified liquor (96) may then befed back to the first conveyor (1).

The twice-pressed solids are conveyed along a third conveyor (52) towarda third roll press (60). The solids are wetted with process orimbibition water (7). The pressate (80) from third press (60) containssmall particles and liquor. This pressate (80) containing smallparticles is added to the feedstock solids feeding the second roll press(50) via third pump (62). This permits leaching of the feedstock with athree-stage countercurrent washing/wetting of the feedstock. Optionally,the pressate (80) may pass through a third swirl tank (63).

At the conclusion of the third press, the feedstock particles have beensheared to a size suitable for pumping. The crushed fiber from the thirdroll press (60) is then sent by conveyor system (84) to a holding bin(86) where it is held for a fixed period of time. The crushed fiber isthen introduced via conveyor (88) to a mix tank (90) where it isslurried to produce a feedstock slurry having a consistency of 8% to20%, or 10% to 18%, or 12% to 15%, or any amount therebetween.Alternatively, the crushed fiber from the third roll press (60) may beslurried in about 4.5 to about 8 parts water per part solid, by weight,or any amount therebetween, for example about 4.5, 4.8, 5.0, 5.2, 5.4,5.6, 5.8, 6.0, 6.2, 6.4, 6.6, 6.8, 7.0, 7.2, 7.4, 7.6, 7.8, or 8.0 partswater per part solid, or any amount therebetween, for example about 5parts.

The slurry is then submitted to further processing (100), which involvesa step of pretreatment in a pretreatment reactor. In the pretreatmentreactor, the slurry is subjected to dilute acid prehydrolysis, and theresulting pretreated slurry is then submitted to enzymatic hydrolysis toconvert the cellulose to sugars as described earlier.

The present invention will be further illustrated in the followingexamples.

EXAMPLES Example 1 Use of Roll Press System to Decrease Particle Size ofWheat Straw

Initial size reduction of wheat straw was performed using a woodchipper-like device. The average length of the straw was reduced toabout 15 cm. The chopped straw was stored until required for presstrials, with about ⅓ in a covered truck bed, and the remainder in anoutdoor bin. Heavy rain was experienced for two days during the trial.

The mill was of a four-roll type, with a “floating” hydraulically dampedtop roll. The clearance between the roll nip points was set to 0.0001″(one ten thousandth of an inch). The rolls were of stainless steelconstruction, 0.76 m (30″) long, with a diameter of 0.457 m (18″). Therolls were circumferentially grooved, with a spacing of 1.3 cm (0.5″)between the grooves. The groove depth was not recorded. The mill wasdriven via a 480 V, 74 A, 60 hp, 1775 rpm electric motor. The motor wasconnected to a gearbox, but the gear reduction ratio and the final rollrpm were not stated. The mill was instrumented with needle gaugesshowing the motor amps and the nip pressure between the rolls.

Straw was manually loaded into a stainless steel wire mesh basket. Asample of the feed straw was obtained from each basket for initialmoisture content measurement, particle size analysis and chemicalcomposition. The basket had a capacity of 1 cubic meter, sufficient for50 kg of loosely packed straw. The exact weight of each basket used wasrecorded. The basket was immersed in approximately 4900 liters of waterin a 5500 liter tank for 20 minutes. The initial and final water levelwere recorded for each basket and a sample of the vat contents obtainedto estimate the amount of water removed with the straw, and to allowcalculation of the amount of straw materials dissolved per basket. Afterthe 20 minute soak, the basket was withdrawn from the vat and allowed todrain for about 10 minutes.

A sample of the soaked and drained straw was taken from each basket formoisture content analysis. Approximately half of the basket (−25 kg ovendried straw) was manually loaded on a 1.5 ft by 8 ft conveyor belt, andthe belt hoisted to the feed hopper on the mill. The speed of the beltwas manually controlled to maintain a level in the feed hopper. Theaverage time required to feed the straw on the belt was measured forfive trials. A sample of the pressed straw was taken for moisturecontent measurement, particle size analysis and chemical composition. Asample of mill pressate was obtained for dissolved and undissolvedsolids analysis. The process was repeated for the second half of thebasket.

Prior to the start of the double pass material, a gas powered water pumpwas obtained, and the leaching vat was emptied and refilled. Single passmaterial was again prepared by processing 8 baskets of straw through asingle mill pass, with the pressed material stored in the plantovernight. The single pass material was then reloaded into baskets,soaked and drained, as previously described, and run through the millfor a second pass.

The triple pass material was produced in a similar fashion.

The single, double, and triple pass straw was analyzed for pump testing.Samples of the feed and pressed straw were also submitted for inorganicand nitrogen analysis. The leachate and pressate were analyzed fordissolved and total solids. In addition, the straw was analyzed formoisture content and particle size.

The particle size was measured in two ways. For the initial wheat strawsample and a sample from each of the mill passes, 100 randomly chosenfibers were measured with a ruler, and the average length calculated.Secondly, portions of the straw were oven dried and a 20 gram sample ofthe oven dried straw were subjected to particle size analysis using aset of five standard sieves, with the amount of straw retained on eachsieve weighed. The mass of material retained on each screen wasrecorded. Three samples each of the coarsely chopped, single, double,and triple milled material were analyzed, and the results averaged foreach type of material.

Medium Consistency (MC®) pump testing was also performed on single,double, and triple pass cane press straw. The double and triple passstraw was combined for analysis. The tests were conducted by adding thewet, pressed straw to a known volume of water in the standpipe. Thetotal water volume in the standpipe was calculated as the sum of theinitial water volume, the seal water leakage rate, and the volume ofwater in the wet straw. Samples of straw were taken from each drum ofmaterial and analyzed for solids content. The drums were weighed, sothat the dry mass of straw could be calculated. The liquid to solidsratio was then calculated as the total water volume divided by the ovendry weight of straw mass.

Power Consumption

The required time for feeding 25 kg oven dried straw was consistentlybetween 20 to 35 seconds, and the average measured time from five trialswas 30 seconds. The dry fiber throughput of the test mill is estimatedto be 2.5-3 tonnes per hour. While feeding straw to the mill, the motorcurrent draw averaged about 55 A, which is 75% of the rated full load,and the nip pressure increased by only 100-200 psig, or about 8%, overthe no load value. The true upper throughput boundary is thus not known.

Using a motor current draw average of 55 A, with a range from 50-57 A,and a motor voltage of 480 V, a feed time of 30 seconds (0.0083 hrs) for25 oven dried kg of straw, the specific power consumption is calculatedas:(55 A*480 V)*(1 KW/1000 W)*0.0083 hr/0.025 tonne straw=8.8 KWhr/tonnestraw.

Note that the Fulton mill has a higher throughput and a lower energyrequirement than the currently used hammermill. The largest mills thatare currently in service were driven by 1200 hp motors, and were capableof processing 9000 tonnes of sugar cane per day.

Particle Length Distribution

The results of the particle length distribution are shown graphically inFIG. 2 for the pressed wheat samples. From observation of the presswhile in operation, it was apparent that a fraction of the straw isbypassing the grooved section of the rolls, and was escaping the millprematurely along the sides of the rolls. This could be avoided byinstallation of steel scrapers along side the rolls. It is this fractionthat is responsible for the oversized material. The fraction ofoversized material is reduced by successive mill passes withoutdramatically changing the average fiber length.

It should be noted that in analysis of particle length distributionusing sieves, the straw is separated on the basis of the particlediameter, not length. While the sizes do not represent the true averagefiber length of the material, previous work has shown a correlationbetween the average particle length and the sieve size for hammermilledstraw. Visual inspection of the sieved samples showed that a similartrend held for the pressed straw material—larger particles were retainedon the larger mesh screens, while the shorter particles were retained onthe smaller screens. The results are presented in FIG. 3, which providesan indication of the distribution of fiber sizes, and the amount offines that are produced.

The results shown in FIGS. 2 and 3 indicate that successive mill passesreduce both the amount of oversize material and the amount of materialretained on the largest sieve. Additionally, the average particle sizetends towards the roll groove spacing of 1.3 cm. Also, the finescontent, classified as material smaller than 0.85 mm, is only slightlyincreased by successive passes through the mill.

The production of milled wheat straw fines is not entirely addressed byFIG. 2 and FIG. 3, since some fraction of the fines will be carried bythe mill pressate. Samples of the pressate were analyzed for undissolvedsolids—that is, wheat straw fines—but the size distribution of thesolids was not determined. Visual inspection of the mill pressateindicated the majority of the wheat straw particles in the pressate werevery small, on the order of 0.2 cm or less. The weight percent ofundissolved solids in the mill pressate is presented in Table 1.

TABLE 1 Weight percent undissolved solids in mill pressate Sample Wt %Undissolved Single pass 0.85 Double pass 1.20 Triple pass 1.70

Using the moisture content of the feed and pressed straw, the totalamount of pressate generated can be calculated. When combined with thedata on the undissolved solids in the pressate, the total amount offines lost in the pressate can be calculated on a dry straw basis. Thisis presented in Table 2.

TABLE 2 Percent of feed straw lost as fines in the pressate Sample %feed as fines in pressate Single pass 2.0 Double pass 2.5 Triple pass3.9

Fines recovery is a standard procedure for the sugar cane industry, andthe normal method is to screen the mill pressate.

Soluble Component Analysis

The composition of the non-structure components of wheat straw is shownin FIG. 4.

The soluble potassium salts show a 90% decrease due to leaching. Thenitrogen content, which is related to the protein content of the straw,is reduced by about 40% after three passes through the cane press. Thesilica, ash and calcium content of the straw are unaffected by theleaching and pressing operation, under the conditions of this test.

Pumpability Testing

Medium Consistency (MC®) pump testing of single, double, and triple passcane press straw was performed. For the single pass material, the finalliquid to solids ratio achieved was 6.1:1. The test was halted when allthe single pass straw had been consumed by the test. The only data thatis available from active instruments during this test are the MC pumpamperages, the standpipe level reading, and the discharge pressure fromMC pump #1 (P20408). Of these, the standpipe level is only of interestin the calculation of the liquid to solids ratio, and the MC pump #1discharge pressure is manually controlled at approximately 65 psig byvarying the pump speed and the gate valve on the MC pump #1recirculation line. The amperage for MC pump #1 is controlled by thesesame means, with a target value close to 100% full load amps. The onlyindependent parameter is the amperage for MC pump #2. The current drawfor the two MC pumps during this trial is presented in FIG. 5. Thecurrent for MC pump #2 increased by only 2%, from 66 to 68% full loadamps (% FLA), during this test.

A similar test was conducted using the combined double and triple passmaterial. The test was halted at a final liquids-to-solids ratio of5.9:1 due to an overflow in the standpipe. Again, it may have beenpossible to push to higher solids consistencies. The current draw forthe two MC pumps is presented in FIG. 6. MC pump #2 shows the same 2%current increase from 66 to 68% FLA.

Summary

Roll press test work was conducted using a Fulton press. The equipmentwas evaluated on the basis of power consumption, throughput, andparticle size reduction. Approximately 300 kg each of single, double andtriple pass material was prepared. Chemical analysis of the straw wasconducted to determine the endogenous salts removed during the pressingoperation.

The wheat straw particle size was reduced from an initial coarselychopped average length of approximately 15 cm to approximately 70% being1.5 cm or less during the first pass, and with the remaining 30% of thestraw having lengths up to 6 cm. Successive passes through the pressreduced the percentage of oversized material, but only slightly reducedthe median size of particles produced. There was a 3-fold increase inthe amount of fines collected in the mill pressate from singly to triplypressed material.

Pump testing of the pressed straw showed behavior similar to hammermilled wheat straw. The final liquid to solids obtained was on the orderof 6:1. The pumpability of the material seemed independent of the numberof cane press passes in that single and double pass material behavedsimilarly.

The average amount of dissolved material in the vat leachate and millpressate was only 2.5% of the initial dry straw mass. Readily solublepotassium salts were removed; calcium, ash, and silica were reduced byless than 10%. The total Kjeldahl nitrogen (TKN) was reduced by about40%.

The estimated throughput of the press used for the testing was 2.5 to 3tonnes oven-dried straw per hour. This rate was based on a feed time ofabout 30 seconds for 25 kg of straw, but the mill may have been runbelow capacity. The specific energy consumption for press operation wascalculated to be on the order of 9 kWhr/tonne of straw. The amount ofpower required to chop the straw to a similar particle length with ahammer mill is 70 kWhr/tonne.

The pressed straw had an average moisture content of 45%. This value wasnot affected by the number of passes through the mill. Previous testingproduced a value of 43% solids with an applied pressure of 2500 psi in ahydraulic press, similar to the average recorded cane press nip pressureof 2900 psi recorded for this trial.

Example 2 Use of Roll Press System with Enhanced Shear to OptimizeParticle Size of Wheat Straw

Initial size reduction of wheat straw is done using a wood chipper-likedevice. The average length of the straw is reduced to about 15 cm. Thechopped straw is stored until required for press trials.

The mill is of a four-roll type, with a “floating” hydraulically dampedtop roll. The clearance between the roll nip points is set to about0.0001″ (one ten thousandth of an inch). The rolls are of stainlesssteel construction, 0.76 m (30″) long, with a diameter of 0.457 m (18″)or 0.432 m (17″). The rolls are circumferentially grooved, with aspacing of 1.3 cm (0.5″) between the grooves. The mill is driven via a480 V, 74 A, 60 hp, 1775 rpm electric motor. The mill is instrumentedwith needle gauges showing the motor amps and the nip pressure betweenthe rolls.

The shear produced by the rolls of the press is increased by either a)using a top roll with a diameter of 0.432 m (17″) compared to 0.457 m(18″) for the bottom rolls, or b) driving the top roll at a speedapproximately 5% faster than that of the bottom rolls. Either of thesesituations results in a speed differential between the rolls, leading toincreased shear applied to the feedstock.

Straw is manually loaded into a stainless steel wire mesh basket. Asample of the feed straw is obtained from each basket for initialmoisture content measurement, particle size analysis and chemicalcomposition. The basket has a capacity of 1 cubic meter, sufficient for50 kg of loosely packed straw. The exact weight of each basket used isrecorded. The basket is immersed in approximately 4900 liters of waterin a 5500 liter tank for 20 minutes. The initial and final water levelare recorded for each basket and a sample of the vat contents obtainedto estimate the amount of water removed with the straw, and to allowcalculation of the amount of straw materials dissolved per basket. Afterthe 20 minute soak, the basket is withdrawn from the vat and allowed todrain for about 10 minutes.

A sample of the soaked and drained straw is taken from each basket formoisture content analysis. Approximately half of the basket (−25 kg ovendried straw) is manually loaded on a 1.5 f by 8 f conveyor belt, and thebelt hoisted to the feed hopper on the mill. The speed of the belt ismanually controlled to maintain a level in the feed hopper. The averagetime required to feed the straw on the belt is measured for five trials.A sample of the pressed straw is taken for moisture content measurement,particle size analysis and chemical composition. A sample of millpressate is obtained for dissolved and undissolved solids analysis. Theprocess is repeated for the second half of the basket.

Prior to the start of the double pass material, the leaching vat isemptied and refilled. Single pass material is again prepared byprocessing 8 baskets of straw through a single mill pass. The singlepass material is then reloaded into baskets, soaked and drained aspreviously described and run through the mill for a second pass.

The triple pass material is produced in a similar fashion.

Samples of the feed and pressed straw are submitted for inorganic andnitrogen analysis. The leachate and pressate are analyzed for dissolvedand total solids. The straw is also analyzed for moisture content andparticle size. The particle size is measured in two ways. For theinitial wheat straw sample and a sample from each of the mill passes,100 randomly chosen fibers are measured with a ruler, and the averagelength calculated. Secondly, portions of the straw are oven dried and a20 gram sample of the oven dried straw are subjected to particle sizeanalysis using a set of five standard sieves, with the amount of strawretained on each sieve weighed. The mass of material retained on eachscreen is recorded. Three samples each of the coarsely chopped, single,double, and triple milled materials are analyzed, and the resultsaveraged for each type of material.

Results obtained using a) a top roll with a diameter of 0.432, and b) atop roll speed approximately 5% faster are similar to those set out inExample 1.

All citations are hereby incorporated by reference.

The present invention has been described with regard to one or moreembodiments. However, it will be apparent to persons skilled in the artthat a number of variations and modifications can be made withoutdeparting from the scope of the invention as defined in the claims.

1. A process for producing ethanol comprising: a) providing a feedstockselected from the group consisting of grasses, cereal straws, stover,and combinations thereof, wherein at least 80% of the feedstock has aparticle length of between about 2 and about 40 cm; b) wetting thefeedstock in an aqueous stream at about 0.25 to about 10 times themaximum water holding capacity of the feedstock to produce a wetfeedstock; c) pressing the wet feedstock through one roll press or aseries of roll presses to remove at least a portion of water and solublesubstances from the wet feedstock and to shear the wet feedstock toproduce a pressed feedstock having a particle size such that, when thepressed feedstock is slurried to produce a slurried feedstock, theslurried feedstock is capable of being pumped at a dry solidsconcentration of about 8% to about 20%, wherein the one roll press, orone or more than one roll press in said series, comprises rolls withcircumferential v-shaped grooves, and wherein the pressed feedstock hasa consistency of at least about 35% dry solids after passing through anip point in the one roll press or a nip point in one or more than oneroll press in said series; d) slurrying the pressed feedstock particlesto produce a slurried feedstock having a dry solids concentration ofabout 8% to about 20% and pumping the slurried feedstock into apretreatment reactor; and e) carrying out dilute acid pretreatment ofthe slurried feedstock, at a temperature of 160° C. to 280° C. toproduce the pretreated feedstock; f) hydrolyzing the pretreatedfeedstock using cellulase enzymes to produce glucose; and g) fermentingthe glucose to produce ethanol.
 2. The process of claim 1, wherein, inthe step of providing (step a), the cereal straw is wheat straw, barleystraw, rice straw, canola straw, or oat straw.
 3. The process of claim1, wherein, in the step of providing (step a), the stover is corn stoveror soybean stover.
 4. The process of claim 1, wherein sand is removedfrom the feedstock prior to pressing (step c).
 5. The process of claim1, wherein, in the step of wetting (step b), the aqueous stream is wateror a water-based solution.
 6. The process of claim 1, wherein, in thestep of providing (step a), the feedstock has a particle length ofbetween about 2 and about 30 cm.
 7. The process of claim 6, wherein, inthe step of slurrying (step d), the slurry is pumped to the pretreatmentreactor and subsequently pretreated at a temperature of 170° C. to 260°C. at pH 0.8 to 2.0 for a period of 0.1 to 30 minutes to produce thepretreated feedstock.
 8. The process of claim 1, wherein the feedstockis partially leached prior to the step of wetting (step b) or pressing(step c).
 9. The process of claim 1, wherein, in the step of pressing(step c), a series of three roll presses are used.
 10. The process ofclaim 1, wherein, in the step of wetting (step b), the liquid is at atemperature of between about 20° C. and about 95° C.
 11. The process ofclaim 9, wherein, in the step of wetting (step b), the liquid is at atemperature of between about 30° C. and about 85° C.
 12. The process ofclaim 1, wherein, in the step of pressing (step c), the one roll pressor one or more than one roll press in said series exerts a pressure ofabout 2400 psi to about 3000 psi on the feedstock.
 13. The process ofclaim 1, wherein, in the step of slurrying (step d), at least 70% of thefeedstock particles are 2.4 cm or less in length.
 14. The process ofclaim 13, wherein, in the step of slurrying (step d), at least 80% ofthe feedstock particles are 2.4 cm or less in length.
 15. The process ofclaim 14, wherein, in the step of slurrying (step d), at least 89% ofthe feedstock particles are 2.4 cm or less in length.
 16. The process ofclaim 1, wherein, in the step of pressing (step c), the one roll pressor one or more than one roll press in said series creates additionalshear to reduce the feedstock particle size.
 17. The process of claim16, wherein the additional shear is created by a difference in diameterof the rolls of the one roll press or at least one roll press in saidseries.
 18. The process of claim 17, wherein the diameter of the rollsof the one roll press, or one or more than one roll press in saidseries, varies by about 1.2 cm to about 20 cm.
 19. The process of claim16, wherein the additional shear is created by a difference in speed ofthe rolls of the one roll press or one or more than one roll press insaid series.
 20. The process of claim 19, wherein the speed of the rollsof the one roll press, or one or more than one roll press in saidseries, varies by about 3.5% to about 10%.
 21. The process of claim 1,in which a series of presses are used, and wherein the step of wetting(step b) comprises countercurrent washing of the feedstock with pressatecollected from one or more than one roll press in said series.
 22. Theprocess of claim 1, wherein, in the step of slurrying (step d), theslurried feedstock has a consistency of between about 10% and about 18%dry solids.
 23. The process of claim 22, wherein the slurried feedstockhas a consistency of between about 12% and about 15% dry solids.
 24. Theprocess of claim 1, wherein, in the step of pressing (step c), a seriesof two roll presses are used.
 25. The process of claim 1, wherein, afterthe step of pressing (step c) and before the step of slurrying (step d),the pressed feedstock has a consistency of at least about 35% drysolids.
 26. The process of claim 1, wherein, in the step of pressing(step c), one or more than one other press or one or more than one otherdewatering device is used in combination with the one roll press or theseries of roll presses, wherein the one or more than one other press orthe one or more than one other dewatering device is not a roll press.27. The process of claim 26, wherein, after the step of pressing (stepc) and before the step of slurrying (step d), the pressed feedstock hasa consistency of at least about 35% dry solids.
 28. The process of claim1, wherein the process is a continuous process with continuous feedingof the feedstock and continuous withdrawal of the pretreated feedstock.